1. Field of Invention
This invention relates to a method and a converter for refining pig-iron into steel, in which oxygen is blown into the melt from below its surface.
2. Description of Prior Art
Even about 100 years ago, the relevant literature mentioned the possibility of blowing pure oxygen instead of air through the converter bottom in the case of bottom-blown converters. The unusually rapid wear in the converter bottom associated with the blowing-in of pure oxygen prevented this proposal from being realized. Moreover, the quality of steel refined with air was adequate for decades, and there was therefore no need to use oxygen instead of air. At a later date, for the purpose of eliminating the harmful action of high nitrogen contents in steel on the welding and cold-working properties of converter-refined steel, the practice of using oxygen-enriched air was initiated. The increase in oxygen-air contamination results in a considerable loss of iron in the form of brown smoke consisting mainly of iron oxide, and the yield was correspondingly low.
Failures in attempts to blow pure oxygen into bottom-blown converters finally resulted in the known oxygenblowing process, in which pure oxygen is blown from above on to the iron melt by a lance. Although this process is very economical for manufacturing ordinary low-carbon steels, it has a number of disadvantages in comparison with the bottom-blown converter. For instance, it requires costly lances which are subject to considerable wear from splashes of iron and slag and from high temperatures in the region of the stream of oxygen impinging on the surface of the bath. Moreover, the mixing of the bath, and thus the equalization of concentrations, with the oxygen-blowing process is not as good and intensive as with the bottom-blown converter. Another disadvantage is that a substantial portion of the oxygen passes to the melt through the slag, the iron-oxide content of which is correspondingly high. This, and the large volume of brown smoke, results in relatively high iron losses. Thus, in the oxygen-blowing (lance) process, the Fe content of the slag amounts to up to 30%, and the iron loss through brown smoke, to about 1.5% of the weight of the steel. This renders necessary costly gas-cleaning installations, to remove the large amounts of iron oxide contained in the waste gas.
Furthermore, in the oxygen-blowing (lance) process, only a portion of the oxygen issuing from the lance is utilized, while a not inconsiderable amount of oxygen reacts with the waste gas of the refining reactions. Since in the said oxygen-blowing process, the carbon contained in the pig-iron is burned only to carbon monoxide, the waste gases still contain about 75% of the theoretical combustion heat of carbon to carbon dioxide. Although for this reason further utilization of these waste gases is advantageous, it is difficult and costly with the said process since, due to the frequent strong ejection of these converters, the gases are hard to collect and must also be subjected to costly cleaning because of the high content of brown smoke. Additional considerable heat losses arise from vaporization of the iron in the region of the burning spot, which is the cause of the brown smoke. Other disadvantages of this process arise from the absence of the stirring motion essential to the homogenization of the melt, usually produced by the blast in conventional bottomblown converters.
Because of the increased iron-oxide enrichment of the slag in this process, and the considerable increase in the amount of slag arising during the refining of a pig-iron rich in phosphorus (basic Bessemer pig-iron), in comparison with the refining of pig-iron poor in phosphorus, it is necessary, to ensure a quiet refining operation, to reduce the silicon and manganese content of the pig-iron far below the usual limits. The manganese content which is basic Bessemer pig-iron amounts, as a rule, to 1% must be reduced to a maximum of 0.6% with the oxygen-lance process, while the silicon content must be reduced to a maximum of 0.3%. This leads to a corresponding restriction in the choice of blast-furnace charge material, and thus to increased costs.
Because of the reduced bath movement in comparison with the basic Bessemer process in the bottom-blown converter, de-phosphorizing with the oxygen-lance process, with a given slag composition, is not as effective as in the bottom-blown basic Bessemer converter. In order to obtain low phosphorus contents, two slags must therefore be used, resulting in a lengthening of the overall process and higher wear in the converter. Furthermore, varying amounts of the first slag, of varying composition, remain behind in the converter, and this affects the course of the second refining operation in an uncontrollable manner. The slag change also brings about considerable heat losses.